Pyrotechnic time delay element

ABSTRACT

A pyrotechnic time delay element which includes a casing made from a plastics material, a pyrotechnic composition inside the volume, a membrane inside the volume against one end of the pyrotechnic composition and a primary explosive inside the volume on an opposing side of the membrane.

BACKGROUND OF THE INVENTION

This invention relates to a time delay element for use in an explosive application.

The ability to control the timing of an explosive detonator forms an integral part of blast design. A time variation of the order of milliseconds can make a difference between a controlled blast with a good result and a complete blast failure with substantial cost implications. These time intervals can be controlled using electric or electronic systems and pyrotechnic delay systems.

A typical chemical detonator system includes components such as a detonator shell which houses a high explosive base charge, a primary explosive charge and a time delay element. Generally the delay element includes an aluminium casing that contains a pyrotechnic delay composition formed from a fuel/oxidiser mixture which, optionally, includes a metal powder or catalyst to control burning characteristics of the composition. Usually the composition is press-loaded into the casing during a manufacturing step.

The aluminium casing is normally manufactured by means of an extrusion process and is relatively expensive. The extrusion process results in a casing of a particular cross sectional shape and this poses a constraint on the shape of the composition which is loaded into the casing. Another factor is that recent detonator developments have been directed to metal-free systems and, from this point of view, the use of aluminium in making a detonator shell is not always desirable.

An object of the present invention is to address, at least to some extent, the aforementioned factors.

SUMMARY OF INVENTION

The invention provides a pyrotechnic time delay element which includes a casing which is formed from a suitable plastics material, the casing comprising a housing inside which is formed a volume, at least one opening to the volume and a pyrotechnic composition which is loaded into the volume through the opening.

The pyrotechnic composition may be consolidated inside the volume after it has been loaded into the volume.

The casing may be formed by means of a suitable moulding process e.g. an injection moulding process.

The plastics material may be of any appropriate kind and may for example be a thermosetting or thermoplastics material which, optionally, is reinforced e.g. by the inclusion of fibres or which is otherwise treated e.g. by radiation, to enhance its mechanical properties.

The plastics material may, itself, consist of a mixture of different injection mouldable materials and, optionally, fibres to enhance its properties so as to increase the strength of the material, to reduce the quantity of material required, or the like.

The housing may be any suitable external cross sectional shape e.g. polygonal or circular, and may be tapered in longitudinal cross section and may have one or more internal or external step formations or the like. The housing may have a closed or sealed end which may be formed by the cap or which may be formed integrally with the remainder of the housing.

The volume, in a longitudinal direction, may have a shape selected from the following: cylindrical, tapered, cylindrical with step formations, and a non-varying cross-sectional shape.

A membrane may be positioned inside the volume separating an ignition end of the casing from the composition. The membrane may have a perforation or opening through it with a shape which, in cross-section, is selected from the following: circular, square, rectangular, cross shaped, star shaped, polygonal.

The perforation or opening in the membrane, in a longitudinal direction of the casing, may vary in cross-sectional area.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which:

FIGS. 1 to 12 are side views in cross section of different casings for use in making a pyrotechnic time delay element, according to the invention,

FIGS. 13 and 14 show, in cross section, different possible shapes of the casing,

FIGS. 15 to 18 show, in cross section, different membranes which can be used in a time delay element of the invention,

FIGS. 19 to 20 are side views of the membrane shown in FIGS. 15 to 18 respectively, and

FIGS. 23 to 27 are side views of other possible membranes which can be used in a time delay element according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is concerned with a non-Metallic alternative to a typical metal time delay element casing. The casing of the invention is preferably made using injection moulding technology using a suitable plastic material or a mixture of plastic materials. The use of injection moulding techniques allows casings of various shapes to be made and also enables inner and outer surfaces of each casing to be varied according to requirement, within reason.

When making the casing of the invention a suitable thermoset or thermoplastic material is selected. A requirement in this respect is that the chosen material should be capable of being injection moulded into an appropriate shape. Thermoset material, if chosen, can be reinforced with carbon or glass fibre to achieve satisfactory strength characteristics. The material can also be treated after moulding to improve its mechanical properties. Similarly, a thermoplastics material, if chosen, can be reinforced with carbon or glass fibre to enhance its strength.

It is also possible to make a custom material by using a mixture of different injection mouldable materials and fillers, optionally with reinforcing, as may be appropriate.

FIG. 1 illustrates from one side and in longitudinal cross-section a typical casing 10 which is made in accordance with the principles of the invention. The casing is moulded from a suitable plastics material and has open opposed ends 12 and 14 respectively.

The casing is filled with a suitable pyrotechnic composition 20 which is placed using any appropriate technique into an interior of the casing. Thereafter, the composition is press-loaded. Substantial force could be used to consolidate the composition in this way and it is essential therefore that the casing 10 must have sufficient strength not to deform during the consolidation process. Apart from this aspect it is to be borne in mind that when the composition is ignited it may then execute an extended timing interval of the order of seconds. The material which is used in the making of the casing must thus be adequately strong and have appropriate properties to enable the material to withstand the effects of the burning composition.

FIG. 2 illustrates a casing 10 which has been press-loaded with a composition 20, as described in connection with FIG. 1, and wherein opposing ends 12 and 14 are sealed by means of respective closures or caps 22 which are forced into the open ends to achieve a tight sealed fit which ensures that the composition 20 is protected against extraneous factors.

FIG. 3 shows the casing 10, again filled with a composition in the manner described in connection with FIG. 1, wherein a cap 28 is engaged over the open end 12 with an external surface of the casing. Similarly in FIG. 4 a second cap 30 is positioned over the opposing end 14.

The casing 12 shown in FIGS. 1 to 4 can be circular in cross section (transverse to a longitudinal axis) as is shown, for example, in FIG. 13. In a variation of the invention, shown in FIG. 14 (by way of example only), the casing 10 has a polygonal shape. FIG. 14 depicts an octagonal shape—again this is exemplary only and non-limiting. If the shape is polygonal then this can be in the form of a regular or an irregular polygon, according to requirement.

FIG. 5 shows a casing 40 which has a generally conical external shape 42 and a substantially similar generally conically tapered volume 46 which, in use, is loaded with a pyrotechnic composition. FIG. 6 shows a casing 48 which has an internal passage 50 of circular cylindrical configuration and an external shape 52 which is tapered. This construction results in one end 54 of the casing having a relatively thick side wall while an opposing end 56 has a thinner side wall.

FIG. 7 shows a casing 60 of compound proportions. The casing includes three sections 62, 64 and 66 respectively each of which has a respective internal volume 62A, 64A and 66A respectively. These volumes differ in size from one another. Additionally, walls of the casing designated 62B, 64B and 66B respectively, vary in thickness and hence in strength.

A different arrangement is embodied in a casing 70 shown in FIG. 8. The casing has an internal volume 72 of circular cylindrical shape and of constant cross section from one end 74 to an opposing end 76 of the casing. However a wall of the casing has a stepped configuration. An initial portion has a thickness 80, an intermediate portion has a thickness 82 which is greater than the thickness 80, and a third section of the casing has a thickness 84 of maximum thickness. Geometrical variations of this kind (i.e. variations in wall thickness, in the length of each wall and in the shape of each passage), and variations such as material types, reinforcements and so on, can all be employed, as appropriate, to obtain specific characteristics in the casing.

FIG. 9 shows a casing 90 which has a regular circular outer shape 92 and an elongate tapered conical passage 94. FIG. 10 shows a casing 96 which has a stepped internal configuration 98 with a regular circular cylindrical external shape 100. FIG. 11 shows a casing 102 which has a tapered external shape 104 with a stepped internal configuration 106.

FIG. 12 illustrates an elongate casing 110 formed in an appropriate, injection-moulding process. The casing, in this example, is in the form of a cylinder of circular cross section. A membrane 112, positioned inside a volume 114, separates an ignition end 116 of the casing from a composition 118 which is placed inside the volume 114. A primary explosive (not shown) is located preferably in the volume, on the ignition side of the membrane—this is the case for all the examples.

The membrane may be solid or it may include at least one aperture or perforation of any appropriate shape or size.

FIG. 15 for example illustrates a membrane 112A which has a conical perforation 120 formed through it. FIG. 19 shows the perforation end-on. FIG. 16 shows a membrane 1126 with a circular perforation 122 and FIG. 20 shows the perforation end-on. FIG. 17 shows a membrane 112C which has a perforation 124 which in cross section is tapered but which, viewed end-on as is shown in FIG. 21, has a square cross section. FIG. 18 shows a membrane 112D with a perforation 126 which, in cross section, is generally circular (see FIG. 22) although, viewed from one side, the perforation has a semi-spherical shape 128. Thus, in an axial direction of the casing, the membrane perforation varies in cross-sectional area.

FIGS. 23 to 27 show different types of formations in various membranes. In FIG. 23 a membrane 130 has a mesh-like configuration. In FIG. 24 a membrane 132 has a slot 134 which is centrally positioned. In FIG. 25 a membrane 136 has a star-like section 138 while, in FIG. 26, a membrane 140 is formed with a perforation or aperture 142 which is in the form of half of an annulus. In FIG. 27 on the other hand a membrane 144 is formed with a cross-shaped perforation 146.

The various membranes and the cross sectional shapes of the perforation or perforations in the membranes can be used, as appropriate, to shape a wave-front which is propagated by the pyrotechnic composition to ensure that optimum ignition of a primary explosive takes place after the time delay element has executed its timing interval.

It is apparent from the aforegoing that a casing, according to the invention, can be made in practically any form, shape or volume with the use of injection moulding techniques. Complex geometrical configurations are possible. These configurations can be used, as appropriate, to ensure that timing intervals are executed with a desired level of accuracy. Additionally, the casings do not make use of metallic sleeves. 

1-11. (canceled)
 12. A pyrotechnic time delay element which includes a casing which is injection-moulded from a non-metallic material, the casing comprising a housing inside which is formed a volume, at least one opening to the volume, a pyrotechnic composition which is loaded into the volume through the opening, and a membrane, inside the volume, which separates an ignition end of the casing from the composition, wherein the membrane has a perforation or opening with a shape, in cross-section, selected circular, square, rectangular, cross-shaped, star-shaped, polygonal, said membrane and cross sectional shape of perforation being used to shape a wave-front, propagated by the pyrotechnic composition, to ensure that optimum ignition of a primary explosive takes place after the time delay element has executed its timing interval.
 13. A time delay element according to claim 12 wherein the pyrotechnic composition is consolidated inside the volume after it has been loaded into the volume.
 14. A time delay element according to claim 12 wherein the non-metallic material is a plastics material selected from the following: i. thermosetting material; ii. thermoplastics material; iii. material which is reinforced by the inclusion of fibers or by means of radiation; iv. mixture of injection-mouldable materials.
 15. A time delay element according to claim 12 wherein the housing has an external cross-sectional shape selected from the following: circular and polygonal; and, in a longitudinal direction, has a shape selected from the following: cylindrical; tapered; and cylindrical with step formations.
 16. A time delay element according to claim 12 wherein opposed ends of the housing are sealed by means of respective closures to protect the pyrotechnic composition which is inside the volume.
 17. A time delay element according to claim 12 wherein the casing has a wall, around the volume, of constant thickness.
 18. A time delay element according to claim 12 wherein the casing has a wall, around the volume, of varying thickness.
 19. A time delay element according to claim 12 wherein the perforation or opening in the membrane, in a longitudinal direction of the casing, varies in cross-sectional area.
 20. A time delay element according to claim 12 wherein the volume, in a longitudinal direction, has a shape selected from the following: cylindrical, tapered, cylindrical with step formations, and a non-varying cross-sectional shape. 